Conventional Batch Distillation

The essential features of a conventional batch distillation (CBD) column (Figure 1.2) are ... 

Abrams, H.J., Miladi, M. and Attarwala, F.T., Preferable alternatives to conventional batch distillation, Presented at the IChemE Symposium Series no. 104, Brighton, 7-9 September, 1987. 

Following sections describe briefly each of these operations for a conventional batch distillation column. 

In general, the optimisation problem to optimise the operation of conventional batch distillation can be stated as follows ... 

An extensive literature survey indicates that the role of column holdup on the performance of batch distillation has been the subject of some controversy until recently. The following paragraphs outline briefly the investigations carried out on column holdup since 1950. Most of the investigations were restricted to conventional batch distillation columns and binary mixtures. The readers are directed to the original work to develop further understanding of the topic. 

Converse and Huber (1965), Robinson (1970), Mayur and Jackson (1971), Luyben (1988) and Mujtaba (1997) used this model for simulation and optimisation of conventional batch distillation. Domenech and Enjalbert (1981) used similar model in their simulation study with the exception that they used temperature dependent phase equilibria instead of constant relative volatility. Christiansen et al. (1995) used this model (excluding column holdup) to study parametric sensitivity of ideal binary columns. 

A liquid binary mixture with Bo = 10 kmol and xbo = <0.6, 0.4> molefraction is subject to conventional batch distillation shown in Figure 4.3. The relative volatility of the mixture over the operating temperature range is assumed constant with a value of (a=) 2. The total number of plates is, N = 20. The vapour boilup rate is, V = 5.0 kmol/hr and the reflux ratio is, r = 0.75. The condenser and total plate holdups are 0.2 and 0.2 kmol respectively. 

Referring to Figure 2.2 for MVC column configuration, the model equations for the rectifying section are the same (except the reboiler equations) as those presented for conventional batch distillation column (Type III, IV, V in section 4.2). The model equations for the stripping section are the same (except the condenser equations) as those presented for inverted batch distillation column (Type III, IV, V in section 4.3.2). However, note that the vapour and liquid flow rates in the rectifying and stripping sections will not be same because of the introduction of the feed plate. 

Seader and Henley (1998) provided a detailed account of the merits and demerits of different integration methods with typical conventional batch distillation examples. They have noted three particular issues with the integration methods applied in batch distillation calculations that require attention. These are the truncation error, stability and stiffness ratio. These will be briefly discussed in the following. 

The optimal operation of a batch column depends of course on the objectives one wishes to achieve at the end of the process. Depending on the objective function and any associated constraints, a variety of dynamic optimisation problems were defined in the past for conventional batch distillation column. Brief formulations of these optimisation problems are presented in the following subsections. Situations in which each formulation can be applied are discussed. 

The minimum time problem is also known as the time optimal control problem. Coward (1967), Hansen and Jorgensen (1986), Robinson (1970), Mayur et al. (1970), Mayur and Jackson (1971), Mujtaba (1989) and Mujtaba and Macchietto (1992, 1993, 1996, 1998) all minimised the batch time to yield a given amount and composition of distillate using conventional batch distillation columns. The time optimal operation is often desirable when the amount of product and its purity are specified a priori and a reduction in batch time can produce either savings in the operating costs of the column itself or permit improved scheduling of other batch operations elsewhere in a process. Mathematically the problem can be written as ... 

Logsdon and Biegler (1993) considered a binary separation of cyclohexane-toluene mixture in a conventional batch distillation column. Maximum distillate problem was considered to maximise the amount of distillate with cyclohexane purity of 0.998 molefraction. The input data for the problem is given in Table 5.7. 

However, conventional batch distillation with chemical reaction (reaction and separation taking place in the same vessel and hence referred to as Batch REActive Distillation- BREAD) is particularly suitable when one of the reaction products has a lower boiling point than other products and reactants. The higher volatility of this product results in a decrease in its concentration in the liquid phase, therefore increasing the liquid temperature and hence reaction rate, in the case of irreversible reaction. With reversible reactions, elimination of products by distillation favours the forward reaction. In both cases higher conversion of the reactants is expected than by reaction alone. Therefore, in both cases, higher amount of distillate (proportional to the increase in conversion of the reactant) with desired purity is expected than by distillation alone (as in traditional approach) (Mujtaba and Macchietto, 1997). 

Barbosa and Doherty (1988) listed a number of chemical reaction schemes which were previously used mainly in continuous distillation. The reaction products in the different reaction schemes considered do not always have lower boiling point than the reactants. The use of conventional batch distillation for those reactions would result in removal of reactants as the distillation proceeds thus lowering conversion and yield of product. Therefore, it is very important to select the right batch distillation column for each type of chemical reaction. 

For example, if all the reaction products are valuable and have lower boiling temperature than the reactants, then conventional batch distillation would be most suitable. As the reaction proceeds the products will be separated in different main-cuts in sequential order. Conversion and yield can be greatly improved in such cases. If only some of the reaction products have low boiling temperature, then a conventional batch column will only remove those products as distillation proceeds. To separate the rest of the products by conventional distillation would require the removal of unreacted reactants from the column first. 

Conventional batch distillation is not suitable when all reaction products have higher boiling temperatures than those of the reactants. Inverted batch distillation is suitable for such situation. If all the reaction products are valuable, as the reaction proceeds, the products will be separated from the bottom of the column in different main bottom cuts in sequential order. 

Unlike the past work, this work focuses on optimal design and operation of multivessel batch distillation column with fixed product demand and strict product specifications. Both the vapour load and number of stages in each column section are optimised to maximise a profit function. For a ternary mixture, the performance of the multivessel column is also evaluated against that of a conventional batch distillation column. Although the profitability and the annual capitalised cost (investment) of the multivessel column is within 2-3% compared to those of conventional column, the operating cost (an indirect measure of the energy cost and environmental impact) is more than 30% lower for multivessel column. Thus, for a given separation task, multivessel column is more environment friendly. 

Batch distillation is an important unit operation used in many chemical industries, and in particular in the manufacture of fine and specialty chemicals. While conventional batch distillation had received much attention, the research in multi-vessel batch distillation (MultiVBD) is handful (Furlonge et al, 1999 Low and Sorenson, 2003, 2005). 

Low and Sorenson (2003) presented the optimal design and operation of MultiVBD column. A profit function based on revenue, capital cost and operating cost was maximized while optimising the number of stages in different column sections, reflux ratio, etc. They compared the performance of MultiVBD with that of conventional batch distillation column for a number of different mixtures and claimed that MultiVBD operation is more profitable. However, for all cases considered in their work, the products specifications and amounts were not matched exactly and therefore the... 

Hot entrainer at the temperature of the column top should be fed continuously at the column head, while a conventional batch distillation takes place. Product, inevitably containing some entrainer, should be withdrawn at the column top and the batch should be continued until there is no further component 1 in the system. The still should be very large in comparison with normal laboratory batch distillation practice, since it will have to hold almost all the entrainer fed into the column head during the course of the batch. 

The column in Figure 4.3a, as explained, is a conventional batch distillation column with the reboiler at the bottom and the condenser at the top. A single column can be used to separate several products using the multi-fraction operation of batch distillation presented in Figure 4.3b. Some cuts may be desired and others may be intermediate products. These intermediate fractions can be recycled to maximize profits and/or minimize waste generation. Figure 4.3c shows a periodic operation... 

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